Thermal dye sublimation transfer also called thermal dye diffusion transfer is a recording method in which a dye-donor element provided with a dye layer containing sublimable dyes having heat transferability is brought into contact with a receiver sheet and selectively, in accordance with a pattern information signal, is heated by means of a thermal printing head provided with a plurality of juxtaposed heat-generating elements or resistors, so that dye is transferred from the selectively heated regions of the dye-donor element to the receiver sheet and forms a pattern thereon, the shape and density of which is in accordance with the pattern and intensity of heat applied to the dye-donor element.
A dye-donor element for use according to thermal dye sublimation transfer usually comprises a very thin support e.g. a polyester support, one side of which has been covered with a dye layer comprising the printing dyes. Usually, an adhesive or subbing layer is provided between the support and the dye layer.
Owing to the fact that the thin support softens when heated during the printing operation and then sticks to the thermal printing head, thereby causing malfunction of the printing apparatus and reduction in image quality, the back of the support (the side opposite to that carrying the dye layer) is typically provided with a heat-resistant layer to facilitate passage of the dye-donor element past the thermal printing head. An adhesive layer may be provided between the support and the heat-resistant layer.
The heat-resistant layer generally comprises a lubricant and a binder. In the conventional heat-resistant layers the binder is either a cured binder as described in e.g. EP 153,880, EP 194,106, EP 314,348, EP 329,117, JP 60/151,096, JP 60/229,787, JP 60/229,792, JP 60/229,795, JP 62/48,589, JP 62/212,192, JP 62/259,889, JP 01/5884, JP 01/56,587, and JP 02/128,899 or a polymeric thermoplast as described in e.g. EP 267,469, JP 58/187,396, JP 63/191,678, JP 63/191,679, JP 01/234,292, and JP 02/70,485).
When multiple prints have to be made using high printing energies in the absence of any cleaning procedures of the thermal printing head, a residue resulting from the binder may form on the heat-generating elements of said thermal printing head and, as a consequence, cause malfunction of the printing device and defects such as jamming, scratching of the printed image, and breakdown of the heat-generating elements. This phenomenon occurs in particular when the average printing power of said heat-generating elements exceeds 4.5 W/mm.sup.2. The average printing power is calculated as the total amount of energy applied during one line time divided by the line time and by the surface area of the heat-generating elements. Conventional thermal printers usually operate with a maximum average printing power of 3 to 4.5 W/mm.sup.2. However, if higher print densities and/or faster printing speeds are wanted, the average printing power has to be higher than 4.5 W/mm.sup.2.
These high printing energies are used in thermal sublimation printers, which for the sublimation (or diffusion) of dye require substantially higher printing energies than thermal wax printers, in which delamination and fusion of the dye layer are caused.
In case silicone-based lubricants are applied in the form of a separate topcoat on the heat-resistant layer for improving the slidability of the dye-donor element past the thermal printing head, a higher amount of deposited residue is formed unfortunately on said thermal printing head.
It has been suggested in e.g. EP 153,880, EP 194,106, EP 279,467, EP 329,117, EP 407,220, and EP 458,538 to incorporate into the heat-resistant layer particles that have a cleaning effect on the thermal printing head during the printing operation. However, soft particles such as organic polymeric beads like e.g. Teflon have no head-cleaning effect during the printing operation. Silicate particles having a Mohs hardness below 2.7 remove dust and loose debris from the surface during the printing operation, but they have no cleaning effect upon thermally degraded polymer, which actually is left on the thermal printing head. This phenomenon is observed especially at high printing energies. Silicate particles having a Mols hardness of at least 2.7 remove dust, loose debris, and thermally degraded polymer, but they have a negative effect on the lifetime of the thermal printing head since they abrade the passivation layer of said head, especially when they are used at high concentration in the heat-resistant layer.